Kinetic friction on a moving object

Hey...this problem arose:

A force of 40N is required to start a 5 kg box moving across a horizontal concrete floor.
A) What is the coefficient of static friction between the box and the floor? (its .82, I got this no problem)

B) If the 40N force continues, the box accelerates at 0.7 m/s^2. What is the coefficient of kinetic friction?

I don't see how to do part B. I know it's .74, but I don't know how. Please explain in detail by showing equation(s).

$$F_{net} = ma = 40 - f$$
$$f = \mu_k mg$$